He says he's calling Trump's bluff. And he's calling Marco Rubio -- quote -- "Trump with a smile." Senator Ted Cruz carpet-bombing the Republican field in a remarkable news conference just three days away from the South Carolina Republican primary.

The battle between privacy and security. Apple refusing to take an extraordinary measure to help the FBI unlock an iPhone belonging to one of the San Bernardino terrorists. And Apple is saying it's for our own protection.

Plus, he's known for his unparalleled affection for humanity, so what got Pope Francis this angry while greeting the faithful?

Good afternoon, everyone. Welcome to THE LEAD. I'm Jake Tapper.

Now, we did warn you that politics in South Carolina tends to get a little nasty, but this is exceeding even our expectations. Donald Trump and Ted Cruz now warning one another, I will see you in court. It began with Trump's objections to a Cruz attack ad. Trump further needled his rival in a statement warning, time will tell, Teddy, whether more lawsuits can follow, perhaps about Cruz's eligibility given his Canadian birth.

Now, in response, Cruz's stinging takedown of his rivals today suggests he may be concerned that their jabs may be swaying some South Carolina voters. And in the midst of all of this bruising competition, a coveted endorsement from the governor of South Carolina, Nikki Haley, is about to be handed down. Hint, it's going to neither of those guys we were just discussing.

CNN correspondent Sunlen Serfaty is live outside a Ted Cruz event in Spartanburg, South Carolina. CNN is hosting the first of two town halls with Republican candidates in the Palmetto State this evening.

really channeling his old days as a lawyer as if he was in a courtroom arguing the case against Donald Trump. He brought with him file folders of evidence, even props. Of course, Donald Trump not handling this attack lightly.

(BEGIN VIDEOTAPE)

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: You can't lie about people like that. It's just incredible.

SERFATY (voice-over): It's a dramatic escalation, Donald Trump and Ted Cruz's fight boiling over. Their war of words now being dominated by legal threats.

SEN. TED CRUZ (R-TX), PRESIDENTIAL CANDIDATE: One of the things I look forward to most of all is deposing Donald Trump.

CRUZ: So, Donald, I would encourage you, if you want to file a lawsuit challenging this ad, claiming it is defamation, file the lawsuit.

SERFATY: At issue, this ad from the Cruz campaign, which attempts to paint Trump as being supportive of abortion rights, using a TV interview of Trump's from 1999.

TRUMP: I am pro-choice in every respect.

NARRATOR: We cannot trust Donald Trump with these serious decisions.

SERFATY: Trump has said he has evolved on the issue and is opposed to abortion rights now. The Trump campaign slapping Cruz with this cease and desist order to try and stop his rival from running the ad.

CRUZ: I have to stay to Mr. Trump, you have been threatening frivolous lawsuits for your entire adult life. Even in the annals of frivolous lawsuits, this takes the cake.

SERFATY: Trump today not backing down, laying into Cruz.

TRUMP: I'm pro-life and he will say I'm pro-choice. And I got a call from a reporter. I hear you're pro-choice. I said, who told you that? Cruz. Cruz. It's unbelievable. No, I'm pro-life. And I say to myself, how can a guy be so dishonest?

SERFATY: Marco Rubio echoing Trump's accusations against Cruz.

SEN. MARCO RUBIO (R-FL), PRESIDENTIAL CANDIDATE: What we have seen in the last few weeks is very disturbing.

SERFATY: Also calling Cruz a liar.

SEN. MARCO RUBIO: Ted unfortunately has proven that he's willing to say or do anything to get elected.

SERFATY: Cruz firing back that Rubio is just taking a page from Trump's playbook.

CRUZ: Marco Rubio is behaving like Donald Trump with a smile.

SERFATY: The infighting within the top tier comes as Donald Trump is coasting above every other candidate in the polls. A new CNN/ORC poll out today shows Trump with a commanding lead in Nevada, 26 points ahead of any other candidate.

TRUMP: Beyond belief, actually. Maybe I don't even have to go there and campaign, I don't know.

SERFATY: Meanwhile, Jeb Bush is still fighting for traction, feeling energized with his family's help, sharpening his attacks.

JEB BUSH (R), PRESIDENTIAL CANDIDATE: With all due respect, Senator Rubio, your four years or five years or whatever it is as senator does not match up to my capabilities of understanding how the world works.

[16:05:08]

SERFATY: But receiving a big blow today, losing out on the endorsement of South Carolina Governor Nikki Haley to Marco Rubio.

(END VIDEOTAPE)

SERFATY: This is a huge get for Marco Rubio. Nikki Haley's endorsement was so coveted by so many within the field. She will formally endorse him tonight in Chapin, South Carolina, and then stay with him out on the...

TAPPER: All right, we lost Sunlen's feed there.

Sunlen Serfaty, thanks.

Let's be sure to tune into CNN tonight for the first of two Republican town halls, where South Carolina voters will get to ask the candidates questions. Tonight features Ben Carson, Marco Rubio and Ted Cruz. You can only see it right here on CNN at 8:00 p.m. Eastern.

Meanwhile, the Democratic race, new questions for Hillary Clinton's campaign today after our latest poll suggests her race against Bernie Sanders in Nevada is much closer than anyone originally thought. Nevada's Democratic caucus is this Saturday, but today Hillary Clinton is already looking ahead to South Carolina.

JOE JOHNS, CNN SENIOR CORRESPONDENT: I think they have been focusing on Nevada in a new way since the weekend for sure, Jake. Look, they say they think it's going to be tight and down to the wire,

but they do point to their organization as a thing at the end of the day that they think will put them over the top.

(BEGIN VIDEOTAPE)

JOHNS (voice-over): Tonight, a Democratic dead heat in Nevada.

SEN. BERNIE SANDERS (VT-I), PRESIDENTIAL CANDIDATE: When we stand together, there is nothing that we cannot accomplish.

JOHNS: Bernie Sanders and Hillary Clinton suddenly locked in a close battle with just three days to go until Saturday's caucuses.

SANDERS: Thank you, New Hampshire.

JOHNS: Sanders is trying to build on his momentum following a big win in last week's New Hampshire primary.

SANDERS: Everything in my political gut tells me that we have the momentum here in this state, that if people come out in large numbers on caucus day, we're going to win here in Nevada.

(CHEERING AND APPLAUSE)

JOHNS: Underneath the Nevada numbers, though, some advantages for Clinton. She's seen as better able to handle foreign policy, immigration and race relations, while Clinton and Sanders are more evenly split on the economy.

HILLARY RODHAM CLINTON (D), PRESIDENTIAL CANDIDATE: We're going to go out and we're going to convince people to caucus on Saturday. Then we're going on to South Carolina. Then we're going on to the March states. Then we're going to wrap up the nomination, and then we're going to win this election.

JOHNS: A Sanders victory in Nevada could upend Clinton's strategy to take command of the Democratic race, as the primary fight shifts to states with more diverse electorates. In Nevada, four in 10 Democrats are nonwhite, and the state is also home to a heavy union presence.

Both groups are crucial when it comes to winning the nomination and in the general election battle this November. Tonight, Clinton is keeping up her outreach to African-American voters, campaigning in Chicago with the mother of Sandra Bland, a Texas woman found dead inside a jail cell following her arrest last year.

CLINTON: My core emphasis and commitment is to knocking down the barriers that hold back African-Americans.

JOHNS: A high-profile Sanders backer, the rapper Killer Mike has stirred controversy over comments he made at an Atlanta rally Tuesday night, repeating what he said was a quote from a woman speaking about her support for Sanders.

MICHAEL "KILLER MIKE" RENDER, MUSICIAN: A few weeks ago, she said Michael, "A uterus doesn't qualify you to be president of the United States."

JOHNS: But the Clinton campaign is seizing on the remark, calling it disappointing.

BRIAN FALLON, CLINTON CAMPAIGN SPOKESMAN: Obviously, if the suggestion there is that Hillary Clinton is asking anyone to vote for her based on her gender, that's completely off-base.

(END VIDEOTAPE)

JOHNS: Hillary Clinton has more some fund-raisers today. Then it's back to Nevada to enter the closing arguments -- Jake, back to you.

TAPPER: All right, Joe Johns in Chicago, thanks so much.

A reminder that while Bernie Sanders is energizing new supporters in these early voting states, Hillary Clinton right now has a lock on endorsements from one very exclusive club, the Senate, where almost all the Democratic senators are backing her. None have endorsed Sanders, except, of course, for Sanders.

Prominent Clinton backer and New Jersey Senator Cory Booker, who's one of those supporters, joins me now. He's the author of "United," a new book charting his meteoric rise from Newark's mayor to senator and all the guidance he got along the way.

Senator, congratulations on the book.

SEN. CORY BOOKER (D), NEW JERSEY: Thank you very much.

TAPPER: Now, I have known you since you ran for mayor of Newark the first time.

BOOKER: Yes. You finally let me at your table.

(LAUGHTER)

TAPPER: Finally. Finally.

I want to get your reaction to what Killer Mike said, because it is causing some consternation on the campaign trail today. He said he was quoting a woman who said, "A uterus does not qualify you to be president."

Sanders' campaign has dismissed objections to this as gotcha politics. The Clinton campaign and some of their supporters seem rather offended. What do you think?

[16:10:01]

BOOKER: You know, look, it's a comment that's distracting us from what the real issues are. There are a lot of folks out there that are struggling in this country.

And it is what it is, but the reality is, this is a contest between Secretary Clinton and Senator Sanders. And I'm excited about Secretary Clinton because she has got the right answers for communities like the ones I live in.

TAPPER: President Obama is considering who to nominate for the Supreme Court, opening -- the vacancy in Scalia -- in the wake of Scalia's demise.

Your name has been floated, as I'm sure you have seen in some of the clips.

BOOKER: Yes.

TAPPER: You have seen that?

BOOKER: I have seen that. I have sent those clips to my mother.

(CROSSTALK)

(LAUGHTER)

TAPPER: If you were asked, would you accept?

BOOKER: I am not focused on that.

I'm going to be the United States senator, riding out my term until 2020. I'm very confident of that fact.

TAPPER: All right. Stick around. We have got a lot more to discuss in the next block.

BOOKER: I'm looking forward to it.

TAPPER: I want to get your thoughts on the Republican field, including Donald Trump's latest attack against the president. Plus, of course, we will talk about your book.

That's right after this very quick break. Stay with us.

BOOKER: Thanks.

(COMMERCIAL BREAK)

TAPPER: Welcome back to THE LEAD.

[16:15:00] We have more in our politics lead. I want to bring back Senator Cory Booker. He just published a new book called "United", which really chronicles all the people in your life who taught you valuable lessons.

I want to get to that in a second, but I do have to ask you a few political questions.

SEN. CORY BOOKER (D), NEW JERSEY: Please?

TAPPER: The Democratic National Committee is they're rolling back the Obama era ban on contributions from federal lobbyists and from PACs. I'm wondering, you're somebody who has extolled good government, whether back when you were mayor of Newark or even as a U.S. senator. Do you think that's a good idea?

BOOKER: I think the campaign finance system is broken. I think -- I've now been in this town for two years and I think it really works against the best interests of government. I think Citizens United led us into reality that I don't think even the Supreme Court justices actually warned, if you read their opinion, they warned about non- disclosure of money, the corrupting influence. So, something has got to change.

Now, does this -- with their decision to do really sort of mirror what the other side is doing? I have no criticism of that in and of itself.

TAPPER: As long as the level -- you think that the level -- of the playing field being level is more important? Because Obama obviously did this as a way of unilateral disarmament, just a way of sending a signal and trying to be a leader on the issue.

BOOKER: Right. And so, I guess my point is I don't like the race to the bottom, which we're doing right now. I really don't. But I can't blame the Democratic Party for trying to compete with the Republican Party, especially -- I remember I was in the Senate during a big budget battle and we were biting for extension of child care tax credit, extension of the earned income tax credit and we were trying to keep things balanced on what they were fighting for, which is all these tax breaks for Wall Street and the like.

I was like, wow, we're in the trenches fighting for working class and poor Americans. They're in the trenches fighting for more tax breaks that are not to the benefit -- it's not like trickle down economics actually works in any way. So, that bothers me that you have the people who benefit from not having that policy --

TAPPER: Yes.

BOOKER: -- or get hurt by not having that policy or further undermines that. So, look, we have to have a serious conversation in this country about radically changing the way we finance campaigns. And I also don't like to think --

TAPPER: But -- then why are you supporting Hillary Clinton? Isn't Bernie Sanders so much more forward-leaning on the campaign finance thing? It's basically that and Wall Street are his two big issues.

BOOKER: That's what bothers me about this campaign. Somehow people are trying to frame it as if Hillary Clinton -- like take the Wall Street plan. I look at objective folks who have analyzed what Hillary Clinton's plan is, better than Bernie Sanders.

TAPPER: Oh, really?

BOOKER: His rhetoric may be fiery and the like, but as far as somebody who actually has the plans and strategies to reform Wall Street, I think if you look at on objective observers, I'd stick with what Hillary Clinton wants to do, intends to do and has a plan to implement. TAPPER: President Obama yesterday in his press conference going after

Donald Trump pretty hard, especially when it comes to Trump's conversations about banning Muslims and the like. Now, you represent New Jersey. Trump said, and we can find no evidence of this ever anywhere, but Trump said that he saw thousands of Muslims celebrating 9/11 on the day of 9/11 in Jersey City. You're from New Jersey and Newark is not far from Jersey City.

What do you say to that?

BOOKER: I'd say it's ridiculous. I brought to the state of the union one of New Jersey's renowned Muslim leaders from Jersey City that was appointed by a Jewish mayor, Chris Christie brought him in, the FBI works with him, knows that community well. Just was not -- that was fiction.

And so, I don't understand, number one, how you can get away with telling such a bold-faced non-truth there. And I also don't understand how we can be at a point in American history where not only the values of our country are under assault, religious -- not just tolerance, but embracing differences, celebrating differences, and basic ideas of human decency and kindness.

For me to watch my governor just drop out of the race and watch him be excoriated for touching and embracing during a storm --

TAPPER: Yes.

BOOKER: -- the president of the United States and we're making it a political deficit that you hug somebody, I mean, that is a low, that's a nadir when such acts of human expression and human kindness are being vilified.

TAPPER: Let's talk about your book, because that's one of the themes of your book, is the idea of bringing people together. One of the things you write about is your work on reforming the justice system.

You've worked with Ted Cruz on that issue. You write about that.

Why do you think he's so unpopular?

BOOKER: Well, I think the point I'm trying to make is that despite somebody's popularity or not, despite whether you agree with somebody's politics or not, we've got to recognize each other for who we are. Our country was founded -- everybody says the Declaration of Independence. But in that is a declaration of interdependence.

You see people pledging to each other, very different people, their lives, their fortunes and their sacred honor. So, if you come to Washington with a sacred obligation, when we swear that oath to our people, it's got to be recognizing that you may be different than me, but I need you.

This is what I don't like about this idea of patriotism when it's used as a cudgel or sword when really love of country must necessitate love of Americans, even of different race, religion or political philosophy.

[16:20:09] And love is a hard way to go.

And so, I've been able to get a lot of things done in the Senate because I don't look at Mike Lee as some Tea Party Republican that I can't -- I look at him first as a human, as a fellow American. And, hey, there's got to be something we can work on together, whether it's Chairman Inhofe, who -- I don't agree with his climate denying and his snowball bringing to the Senate floor. I could go back to the floor and shout him down and score points with my base, or I could say, hey, let's work on something like we did to an amendment to an education bill on homeless children and foster kids. So --

TAPPER: And people out there are wondering, this is actually what he's like off camera too. You love to pivot to humanity. You are one of the most optimistic people I've ever met.

BOOKER: Well, because as I say in my book, the greatest heroes I've known are not the people that make headlines, but they heal their communities, they build bridges and they do things other people think can't be done because they create uncommon connections with others.

TAPPER: All right. Senator Cory Booker, the book is "United." Good luck with it. Thanks for coming.

BOOKER: It's great to be on.

TAPPER: It's been a long time.

BOOKER: Before I got down here, you used t take -- you took me out, we had a nice drink. Now, you ignore me down here in Washington.

TAPPER: That's not true at all.

(CROSSTALK)

BOOKER: Backbenching freshman --

TAPPER: You know that's not true at all.

BOOKER: Are you calling me a liar?

(CROSSTALK)

TAPPER: No, it's an untruth. It's an untruth.

BOOKER: It's an untruth.

TAPPER: Thank you so much. Good to see you.

Apple is saying that hacking a terrorist cell phone could lead to all your secrets getting out. The battle between big tech and big government with you in the middle, it could go all the way to the Supreme Court.

Plus, he's the guy who made so many rediscover the power of a smile. So, what did one worshipper do to make Pope Francis scowl like this?

(COMMERCIAL BREAK)

[16:26:09] TAPPER: Welcome back to THE LEAD. I'm Jake Tapper.

The national lead now: tech giant Apple is taking some heat for fighting a court order that would force it to help the FBI break into a cell phone once used by one of the San Bernardino terrorists. The government wants to scour that device for any information about other terrorists who might pose a danger to innocent people, like the 14 killed in San Bernardino in December.

To give you a sense of the sharply divided debate, Arkansas Senator Tom Cotton, a Republican and a veteran, said this in part, quote, "Apple chose to protect a dead ISIS terrorist's privacy over the security of the American people."

Now, on the other hand, Edward Snowden tweeted this, "The FBI is creating a world where citizens rely on Apple to defend their rights rather than the other way around."

CNN tech correspondent Laurie Segall joins me now.

Now, Laurie, the government only wants to unlock this Apple -- this Apple phone, but Apple is warning creating the ability to get into this device poses a risk for everyone.

LAURIE SEGALL, CNN MONEY TECH CORRESPONDENT: Absolutely. That's what's fascinating about this. We've had this debate before, privacy versus protection, where do you weigh in. But this particular case, Jake, this case has the ability to set a major precedent.

(BEGIN VIDEOTAPE)

SEGALL (voice-over): It's a move Apple CEO Tim Cook is calling unprecedented and a threat to the security of customers. A judge is ordering Apple to help the FBI break into a cell phone used by one of the San Bernardino shooters. The FBI is asking to disable an iPhone security feature that locks out users and erases the phone's data after the wrong password is entered ten times.

Cook says that would mean building a new operating system with the potential to unlock any iPhone. He says if the FBI could use it as a backdoor into your iPhone, hackers could too.

He's spoken openly about this in the past.

TIM COOK, CEO, APPLE: There have been people that suggest that we should have a backdoor. But the reality is if you put a backdoor in, that backdoor is for everybody, for good guys and bad guys.

INTERVIEWER: Help me understand how you get to the government's dilemma.

COOK: I don't believe that the trade-off is privacy --

INTERVIEWER: Versus security.

COOK: -- versus national security.

SEGALL: Today, the White House spokesman disputed it would open the door to all phones.

JOSH EARNEST, WHITE HOUSE PRESS SECRETARY: They are not asking Apple to redesign its product or to create a new backdoor to one of their products. They're simply asking for something that would have an impact on this one device.

SEGALL: Using other means like cell phone tower data, the FBI has been able to extract location data from the shooter's phone. They are hoping to gain more valuable information.

But security experts say this particular move could have implications for regular civilians.

DAVID KENNEDY, SECURITY RESEARCHER: This basically reduces the security over all iPhone devices and platforms and wouldn't be the most secure platform out there today.

SEGALL: Putting Apple in the position of choosing between the fight of terrorism and the fight against hackers. Given Apple's choice it's not only facing a legal fight but a reputation battle that's even hit the campaign trail.

DONALD TRUMP (R), PRESIDENTIAL CANDIDATE: To think that Apple won't allow us to get into her cell phone, who do they think we are? Now, we have to open it up. This is one case and this is a case that certainly we should be able to get into the phone.

SEN. MARCO RUBIO (R-FL), PRESIDENTIAL CANDIDATE: If you passed a law in the U.S. that says we don't allow encryption, there's already existing software that will remain in place and we can't control what foreign countries, what's designed in other countries.

SEGALL: Bringing center stage on an election year a debate on privacy versus protection, more relevant than ever in the digital age.

(END VIDEOTAPE)

SEGALL: Jake, a teeny bit of background on this, you had the Snowden revelations years ago and there was a lot -- and people were very surprised on how much oversight -- how much government oversight they had into the tech companies and how much data was being handed over.

So, tech companies immediately distanced themselves and they have been very -- they have been very publicly nervous about saying, you know, we are helping out. But in many of these cases they do help out. But now, we're seeing this all come to a head. We're seeing Washington and Silicon Valley have to play nice together and you're going to have to see this debate come out in the public. It will be interesting to see how it plays out.